A heat sink is a device that is attached to a microprocessor chip to keep it from overheating by conducting the heat generated from the chip to the ambient environment which may be air or a liquid coolant. Basic heat sink structures include a heat spreader which makes thermal contact with the silicon chip via an interface of a thermally conductive adhesive and a set of fins which provide for conduction of the heat from the chip to the ambient environment. The purpose of the heat spreader is to provide good thermal conduction of heat emanating from the chip area to a larger area of the heat sink. For air cooled heat sinks, the larger area would comprise a plurality of fins which may be made of copper or aluminum to transfer the heat to the ambient air.
The thickness and reliability of the thermal interface is determined by a number of factors, including mechanical deformation of the package stack. There are many types of thermal interface materials used, including thermal pastes, liquids, epoxies, and metals.
In the case of paste thermal interfaces, changes in the thermal interface gap due to bowing of the chip or spreader can lead to paste pumping and thermal interface failure. In the case of epoxy or metal interfaces the coefficient of thermal expansion (CTE) mismatch between the heat spreader and the chip results in stress on the interface which may lead to adhesion failures. The stiffness of the spreader itself can also contribute to internal stresses.
Most computers or microprocessors sold today will have a heat sink already attached to the chip. This combination of the chip carrier, chip and heat sink is often referred to as the “chip package.” The basic design of a chip package is shown in FIG. 1 in which a non-compliant heat spreader 108 makes thermal contact with the chip 104 through a thermal interface material 106. The chip 104 makes contact with the carrier substrate 102 via solder balls 110 which produce electrical and mechanical connections between the chip 104 and the substrate 102.
In addition, epoxy underfill is used to create a mechanical bond between the chip and the substrate to reduce the mechanical forces on the solder balls 110. There are a number of stresses induced on the thermal interface 106 as a result of the CTE mismatch between the heat spreader 106, the chip 104 and the chip carrier 102. In some cases the heat spreader may be mechanically attached to the carrier, which can result in bowing of the chip package and generates forces and dimensional changes of the thermal interface gap.
Currently produced heat sinks fail to provide for the structural stresses and strains generated during the operation of the electronic device. There is a need for a heat spreader which provides high thermal conductivity and compliance in all directions and provides a means to enable thermal conduction while maintaining a very small or zero gap between the chip and heat spreader.